Place metallic trivet in the instant pot, add the stock to the bottom of the pot. Then place the brisket over the trivet. Add garlic, pickling spice, garlic, onion, and bay leaves.
Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL-HIGH setting and set the cooking time for 70 minutes at high pressure. The Instant Pot will take around 10 minutes to come to pressure and start cooking. When the cooking program ends, allow the steam to naturally release for 10 minutes, then quickly release the rest of the steam.
Carefully remove the lid, and remove the corned beef onto a plate. Tent with foil to keep it warm.
Remove the trivet from the instant pot, then add the potatoes, carrots, and cabbage. Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL-HIGH setting and set the cooking time for 3 minutes at high pressure. When the cooking program ends, quickly release the steam.
Meanwhile, as the vegetables are being cooked make the mustard glaze by combining mustard with sugar and brush it over the corned beef.
Remove the cooked vegetables from the instant pot, and serve with the beef. Garnish with chopped parsley.